In numerous thermodynamic processes and heat treatments, such as, for example, furnaces for the reduction of metal oxides or for the annealing of rolled steel sections, operating conditions are used which require an atmosphere produced by the incomplete combustion of the fuel present or introduced into the furnace itself. This type of operation has a certain number of disadvantages.
Above all, it is necessary, on the one hand, to pass said fumes to a post-combustion device in order to avoid a discharge of seriously polluting unburned gases to the atmosphere and, on the other hand, the significant calorific value of the fumes themselves is not directly exploited for the process. The recovery of said heat by preheating the combustion air by means of an air/fumes exchanger is as a rule modest because of the limitation of the maximum working temperature of the metal materials used for said exchangers, which as a rule does not allow air-preheating temperatures higher than 500-600.degree. C. In addition, both the discharge conduits for the fumes and those for providing the air carry gas streams at a high temperature, which therefore necessitate extensive insulation.